1. Field of the Invention
This invention relates to colour imaging of security document precursors with the aim of producing security documents that are difficult to falsify.
2. Description of the Related Art
Articles are laser marked in order to ensure product safety and authenticity. For example, packaging material of pharmaceuticals is laser marked to enable a consumer to verify the genuineness of a product. Laser marked security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various papers or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such articles and security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking an article, a colour change is observed by local heating of material in the bulk of the article, while in laser engraving material is removed by ablation.
Today, laser marking employed in the manufacture of security documents consists solely of a “black” laser marking method via the carbonization of a polymer, usually polycarbonate as disclosed in e.g. EP 2181858 A (AGFA). There has been considerable interest in being able to produce colour images through laser marking.
U.S. Pat. No. 4,720,449 (POLAROID) discloses a thermal imaging method for producing colour images on a support carrying at least one layer of a colourless compound, such as di- or triarylmethane, by conversion of electromagnetic radiation into heat. The laser beams may have different wavelengths in a range above 700 nm with at least about 60 nm apart so that each imaging layer having a different infrared absorber may be exposed separately to convert a colourless triarylmethane compound into a coloured form.
WO 2009/106036 A (BUNDESDRUCKEREI) discloses a method for producing a security and value document containing a polymer laminate formed from a basic polymer laminate and a polymer coating. The polymer coating has a printable substrate layer that can absorb a diffusible ink printed by thermosublimation or inkjet printing. The polymer laminate can be laser marked through pyrolysis (carbonization).
EP 2181851 A (AGFA) discloses that a dye image in a dye diffusion transfer image receptor layer comprising a polymeric mordant could be laser engraved, while no laser engraving appeared to be possible with dye images obtained with other imaging techniques such as thermal dye sublimation and inkjet.
EP 2332738 A (AGFA) discloses the laser perforation of a security document precursor with two or more holes near a perimeter edge surface of the precursor, wherein at least one of the holes is filled with a coloured material by e.g. inkjet ink printing, so that the coloured material is visible from a direction perpendicular on the perimeter edge surface.
US 2009201321 A (XEROX) discloses a system integrating a digital press with an ink jet device to form a security document including a xerographic portion and an invisible inkjet ink portion, in order to produce documents in a continuous inline process.
Since methods for falsification and counterfeiting of security documents also continue to develop and improve, it remains a constant battle to protect security documents against falsification and counterfeiting by developing new security features and methods of securing such documents. There remains a need for multicolour laser markable articles which are difficult to falsify.